1. Field of the Invention
This invention relates to hydro-pneumatic vehicle suspension systems, and in particular to such systems having discrete hydraulic circuits, each of which is pressurized with a gas-charged accumulator.
2. Discussion of the Prior Art
Hydro-pneumatic vehicle suspension systems are well known. Typically, one or more single or double acting hydraulic cylinders are provided at each wheel to support the vehicle, with hydraulic circuits interconnecting the cylinders. A gas-charged accumulator is typically used to pressurize the hydraulic circuits and therefore, provide resilient displacement of the cylinders and consequently springing of the vehicle.
One hydro-pneumatic vehicle suspension system has come to be known as the "X" type. In such a system, the cylinders of diagonally opposite wheels of a vehicle are connected with discrete hydraulic circuits, each circuit connecting the bore side of one of the cylinders with the rod side of the diagonally opposite cylinder. As used herein, "discrete" as referring to a hydraulic circuit means that the hydraulic circuit is closed against hydraulic fluid flow to or from another circuit, except for normal leakage, and includes a hydraulic circuit which is normally closed from external communication by a valve, but can be opened by the valve to an external source or depository of hydraulic fluid. Such an X-system is known, for example, from patents such as French Patent No. 1,535,641 and U.S. Pat. Nos. 4,270,771; 5,447,332; 5,562,305; 5,601,306; and 5,601,307.
Thus, in an X-system, there are four discrete hydraulic circuits, with one pair of circuits associated with one set of diagonally opposite wheel support cylinders and the other pair associated with the other set of diagonally opposite wheel support cylinders. Neglecting friction of the cylinders, the two circuits of each pair are at substantially the same pressure since that pressure is communicated from one discrete hydraulic circuit to the other through the pistons of the cylinders
It is also known that it is desirable to equalize the pressures between the two pairs of discrete hydraulic circuits so that the wheel loading is kept substantially constant. In U.S. Pat. No. 4,270,771, pressure equalization between the two pairs of discrete hydraulic circuits is obtained by making the cylinders identical and by providing inlet/outlet check valves for each cylinder which supply all the cylinders with the same pressure or exhaust all the cylinders to the same tank pressure. In U.S. Pat. No. 5,447,332 and other patents listed above, pressure is equalized from one pair of circuits to the other by use of a "load distribution unit" (or "LDU") which has four chambers separated from fluid communication with one another by pistons, and each discrete hydraulic circuit is communicated with one of the chambers so as to substantially equalize the pressures among the four discrete hydraulic circuits.